The present invention relates to a gallium nitride based semiconductor laser device, and more particularly to a gallium nitride based semiconductor laser device with an optimized optical confinement structure.
Gallium nitride has a larger energy band gap than other typical compound semiconductors such as indium phosphide and gallium arsenide, for which reason gallium nitride based compound semiconductor is attractive for application to a light emitting device such as semiconductor laser for emitting a light having a wavelength in the range of green to ultraviolet ray.
Six cases of gallium nitride based semiconductor lasers have been reported. In every cases, the active layer comprises a single or multiple quantum well structure of In.sub.x Ga.sub.1-x N quantum well layers and In.sub.x Ga.sub.1-x N barrier layers having a larger energy band gap than an energy band gap of the In.sub.x Ga.sub.1-x N quantum well layers. Optical guide layers are made of gallium nitride. Further, an Al.sub.x Ga.sub.1-x N layer is provided on the In.sub.x Ga.sub.1-x N active layer for prevention of dissociation and evaporation of indium from the In.sub.x Ga.sub.1-x N active layer. The active region comprises the active layer, the optical guide layer and the Al.sub.x Ga.sub.1-x N layer. Cladding layers are provided to sandwich the active region. The cladding layers are made of Al.sub.x Ga.sub.1-x N (0.05.ltoreq.x&lt;0.15) and has a thickness of not more than 0.5 micrometers.
FIG. 1 is a fragmentary cross sectional elevation view illustrative of a gallium nitride based semiconductor laser device in a first prior art. The conventional gallium nitride based semiconductor laser device is formed on a (0001)-face of a sapphire substrate 101. An undoped GaN buffer layer 102 is provided on the (0001)-face of a sapphire substrate 101. The undoped GaN buffer layer 102 has a thickness of 300 .ANG.. An n-type GaN contact layer 103 is provided on the undoped GaN buffer layer 102. The n-type GaN contact layer 103 is doped with Si. The n-type GaN contact layer 103 has a thickness of 3 .mu.m. An n-type In.sub.0.1 Ga.sub.0.9 N layer 104 is provided on the n-type GaN contact layer 103. The n-type In.sub.0.1 Ga.sub.0.9 N layer 104 is doped with Si. The n-type In.sub.0.1 Ga.sub.0.9 N layer 104 has a thickness of 0.1 .mu.m. An n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405 is provided on the n-type In.sub.0.1 Ga.sub.0.9 N layer 104. The n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405 is doped with Si. The n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405 has a thickness of 0.4 .mu.m. An n-type GaN optical guide layer 106 is provided on the n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405. The n-type GaN optical guide layer 106 is doped with Si. The n-type GaN optical guide layer 106 has a thickness of 0.1 .mu.m. A multiple quantum well active layer 107 is provided on the n-type GaN optical guide layer 106. The multiple quantum well active layer 107 comprises 26 periods of 25 .ANG.-thick undoped In.sub.0.2 Ga.sub.0.8 N quantum well layers and 50 .ANG.-thick undoped In.sub.0.50 Ga.sub.0.95 N barrier layers. A p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is provided on the multiple quantum well active layer 107. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is doped with Mg. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 has a thickness of 200 .ANG.. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is capable of suppressing dissociation and evaporation of indium from the indium gallium nitride layers of the multiple quantum well active layer 107. A p-type GaN optical guide layer 109 is provided on the p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. The p-type GaN optical guide layer 109 is doped with Mg. The p-type GaN optical guide layer 109 has a thickness of 0.1 .mu.m. A p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110 is provided on the p-type GaN optical guide layer 109. The p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110 is doped with Mg. The p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110 has a thickness of 0.4 .mu.m. A p-type GaN contact layer 111 is provided on the p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110. The p-type GaN contact layer 111 is doped with Mg. The p-type GaN contact layer 111 has a thickness of 0.5 .mu.m. A p-electrode 112 is provided on the p-type GaN contact layer 111. The p-electrode 112 comprises laminations of a nickel layer and a gold layer. An n-electrode 113 is provided on a part of the n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer and an aluminum layer. The above gallium nitride based semiconductor laser is disclosed in Japan Journal of Applied Physics 35 (1996) L74.
FIG. 2 is a fragmentary cross sectional elevation view illustrative of a gallium nitride based semiconductor laser device in a second prior art. The conventional gallium nitride based semiconductor laser device is formed on a (11-20)-face of a sapphire substrate 501. An undoped GaN buffer layer 102 is provided on the (11-20)-face of a sapphire substrate 501. The undoped GaN buffer layer 102 has a thickness of 500 .ANG.. An n-type GaN contact layer 103 is provided on the undoped GaN buffer layer 102. The n-type GaN contact layer 103 is doped with Si. The n-type GaN contact layer 103 has a thickness of 3 .mu.m. An n-type In.sub.0.1 Ga.sub.0.9 N layer 104 is provided on the n-type GaN contact layer 103. The n-type In.sub.0.1 Ga.sub.0.9 N layer 104 is doped with Si. The n-type In.sub.0.1 Ga.sub.0.9 N layer 104 has a thickness of 0.1 .mu.m. An n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505 is provided on the n-type In.sub.0.1 Ga.sub.0.9 N layer 104. The n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505 is doped with Si- The n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505 has a thickness of 0.4 .mu.m. An n-type GaN optical guide layer 106 is provided on the n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505. The n-type GaN optical guide layer 106 is doped with Si. The n-type GaN optical guide layer 106 has a thickness of 0.1 .mu.m. A multiple quantum well active layer 507 is provided on the n-type GaN optical guide layer 106. The multiple quantum well active layer 507 comprises 20 periods of 25 .ANG.-thick undoped In.sub.0.2 Ga.sub.0.8 N quantum well layers and 50 .ANG.-thick undoped In.sub.0.05 Ga.sub.0.95 N barrier layers. A p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is provided on the multiple quantum well active layer 507. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is doped with Mg. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 has a thickness of 200 .ANG.. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is capable of suppressing dissociation and evaporation of indium from the indium gallium nitride layers of the multiple quantum well active layer 507. A p-type GaN optical guide layer 109 is provided on the p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. The p-type GaN optical guide layer 109 is doped with Mg. The p-type GaN optical guide layer 109 has a thickness of 0.1 .mu.m. A p-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 510 is provided on the p-type GaN optical guide layer 109. The p-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 510 is doped with Mg. The p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110 has a thickness of 0.4 .mu.m. A p-type GaN contact layer 111 is provided on the p-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 510. The p-type GaN contact layer 111 is doped with Mg. The p-type GaN contact layer 111 has a thickness of 0.5 .mu.m. A p-electrode 112 is provided on the p-type GaN contact layer 111. The p-electrode 112 comprises laminations of a nickel layer and a gold layer. An n-electrode 113 is provided on a part of the n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer and an aluminum layer. The above gallium nitride based semiconductor laser is disclosed in Japan Journal of Applied Physics 35 (1996) L217.
FIG. 3 is a fragmentary cross sectional elevation view illustrative of a gallium nitride based semiconductor laser device in a third prior art. The conventional gallium nitride based semiconductor laser device is formed on a (111)-face of a MgAl.sub.2 O.sub.4 substrate 601. An undoped GaN buffer layer 102 is provided on the (111)-face of a MgAl.sub.2 O.sub.4 substrate 601. The undoped GaN buffer layer 102 has a thickness of 300 .ANG.. An n-type GaN contact layer 103 is provided on the undoped GaN buffer layer 102. The n-type GaN contact layer 103 is doped with Si. The n-type GaN contact layer 103 has a thickness of 3 .mu.m. An n-type In.sub.0.1 Ga.sub.0.9 N layer 104 is provided on the n-type GaN contact layer 103. The n-type In.sub.0.1 Ga.sub.0.9 N layer 104 is doped with Si. The n-type In.sub.0.1 Ga.sub.0.9 N layer 104 has a thickness of 0.1 .mu.m. An n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505 is provided on the n-type In.sub.0.1 Ga.sub.0.9 N layer 104. The n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505 is doped with Si. The n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505 has a thickness of 0.4 .mu.m. An n-type GaN optical guide layer 106 is provided on the n-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 505. The n-type GaN optical guide layer 106 is doped with Si. The n-type GaN optical guide layer 106 has a thickness of 0.07 .mu.m. A multiple quantum well active layer 607 is provided on the n-type GaN optical guide layer 106. The multiple quantum well active layer 607 comprises 20 periods of 25 .ANG.-thick undoped In.sub.0.15 Ga.sub.0.85 N quantum well layers and 50 .ANG.-thick undoped In.sub.0.05 Ga.sub.0.95 N barrier layers. A p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is provided on the multiple quantum well active layer 507. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is doped with Mg. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 has a thickness of 200 .ANG.. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is capable of suppressing dissociation and evaporation of indium from the indium gallium nitride layers of the multiple quantum well active layer 607. A p-type GaN optical guide layer 109 is provided on the p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. The p-type GaN optical guide layer 109 is doped with Mg. The p-type GaN optical guide layer 109 has a thickness of 0.1 .mu.m. A p-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 510 is provided on the p-type GaN optical guide layer 109. The p-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 510 is doped with Mg. The p-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 510 has a thickness of 0.4 .mu.m. A p-type GaN contact layer 111 is provided on the p-type Al.sub.0.12 Ga.sub.0.88 N cladding layer 510. The p-type GaN contact layer 111 is doped with Mg. The p-type GaN contact layer 111 has a thickness of 0.4 .mu.m. A p-electrode 112 is provided on the p-type GaN contact layer 111. The p-electrode 112 comprises laminations of a nickel layer and a gold layer. An n-electrode 113 is provided on a part of the n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer and an aluminum layer. The above gallium nitride based semiconductor laser is disclosed in Japan Journal of Applied Physics 68 (1996) 2405.
FIG. 4 is a fragmentary cross sectional elevation view illustrative of a gallium nitride based semiconductor laser device in a fourth prior art. The conventional gallium nitride based semiconductor laser device is formed on a (0001)-face of a sapphire substrate 101. An undoped AlN buffer layer 702 is provided on the (111)-face of the sapphire substrate 101. The undoped AlN buffer layer 702 has a thickness of 300 .ANG.. An n-type GaN contact layer 103 is provided on the undoped AlN buffer layer 702. The n-type GaN contact layer 103 is doped with Si. The n-type GaN contact layer 103 has a thickness of 3 .mu.m. An n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405 is provided on the n-type GaN contact layer 103. The n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405 is doped with Si. The n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405 has a thickness of 0.5 .mu.m. An n-type GaN optical guide layer 106 is provided on the n-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 405. The n-type GaN optical guide layer 106 is doped with Si. The n-type GaN optical guide layer 106 has a thickness of 0.1 .mu.m. A single quantum well active layer 707 is provided on the n-type GaN optical guide layer 106. The single quantum well active layer 707 comprises a 15 .ANG.-thick undoped In.sub.0.1 Ga.sub.0.9 N quantum well layer. A p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is provided on the single quantum well active layer 707. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is doped with Mg. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 has a thickness of 200 .ANG.. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is capable of suppressing dissociation and evaporation of indium from the indium gallium nitride layers of the single quantum well active layer 707. A p-type GaN optical guide layer 109 is provided on the p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. The p-type GaN optical guide layer 109 is doped with Mg. The p-type GaN optical guide layer 109 has a thickness of 0.1 .mu.m. A p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110 is provided on the p-type GaN optical guide layer 109. The p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110 is doped with Mg. The p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110 has a thickness of 0.5 .mu.m. A p-type GaN contact layer 111 is provided on the p-type Al.sub.0.15 Ga.sub.0.85 N cladding layer 110. The p-type GaN contact layer 111 is doped with Mg. The p-type GaN contact layer 111 has a thickness of 0.8 .mu.m. A p-electrode 112 is provided between silicon oxide layers 714 provided on the p-type GaN contact layer 111. The p-electrode 112 comprises laminations of a nickel layer and a gold layer. An n-electrode 113 is provided on a part of the n-type GaN contact layer 103 The n-electrode 113 comprises a titanium layer and an aluminum layer. The above gallium nitride based semiconductor laser is disclosed in Electric Letters 32 (1996) 1105.
FIG. 5 is a fragmentary cross sectional elevation view illustrative of a gallium nitride based semiconductor laser device in a fifth prior art. The conventional gallium nitride based semiconductor laser device is formed on a (11-20)-face of a sapphire substrate 601. An undoped GaN buffer layer 102 is provided on the (11-20)-face of the sapphire substrate 601. The undoped GaN buffer layer 102 has a thickness of 300 .ANG.. An n-type GaN contact layer 103 is provided on the undoped GaN buffer layer 102. The n-type GaN contact layer 103 is doped with Si. The n-type GaN contact layer 103 has a thickness of 3 .mu.m. An n-type In.sub.0.05 Ga.sub.0.95 N layer 804 is provided on the n-type GaN contact layer 103. The n-type In.sub.0.05 Ga.sub.0.95 N layer 804 is doped with Si. The n-type In.sub.0.05 Ga.sub.0.95 N layer 804 has a thickness of 0.1 .mu.m. An n-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 805 is provided on the n-type In.sub.0.05 Ga.sub.0.95 N layer 804. The n-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 805 is doped with Si. The n-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 805 has a thickness of 0.4 .mu.m. An n-type GaN optical guide layer 106 is provided on the n-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 805. The n-type GaN optical guide layer 106 is doped with Si. The n-type GaN optical guide layer 106 has a thickness of 0.1 .mu.m. A multiple quantum well active layer 807 is provided on the n-type GaN optical guide layer 106. The multiple quantum well active layer 807 comprises 7 periods of 25 .ANG.-thick undoped In.sub.0.2 Ga.sub.0.8 N quantum well layers and 50 .ANG.-thick undoped In.sub.0.05 Ga.sub.0.95 N barrier layers. A p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is provided on the multiple quantum well active layer 807. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is doped with Mg. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 has a thickness of 200 .ANG.. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is capable of suppressing dissociation and evaporation of indium from the indium gallium nitride layers of the multiple quantum well active layer 807. A p-type GaN optical guide layer 109 is provided on the p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. The p-type GaN optical guide layer 109 is doped with Mg. The p-type GaN optical guide layer 109 has a thickness of 0.1 .mu.m. A p-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 810 is provided on the p-type GaN optical guide layer 109. The p-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 810 is doped with Mg. The p-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 810 has a thickness of 0.4 .mu.m. A p-type GaN contact layer 111 is provided on the p-type Al.sub.0.07 Ga.sub.0.93 N cladding layer 810. The p-type GaN contact layer 111 is doped with Mg. The p-type GaN contact layer 111 has a thickness of 0.2 .mu.m. A p-electrode 112 is provided on the p-type GaN contact layer 111. The p-electrode 112 comprises laminations of a nickel layer and a gold layer. An n-electrode 113 is provided on a part of the n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer and an aluminum layer. The above gallium nitride based semiconductor laser is disclosed in Extended Abstracts of 1996 International Conference on Solid State Device and Materials, Yokohama, 1996, pp. 67-69.
FIG. 6 is a fragmentary cross sectional elevation view illustrative of a gallium nitride based semiconductor laser device in a sixth prior art. The conventional gallium nitride based semiconductor laser device is formed on a (11-20)-face of a sapphire substrate 601. An undoped GaN buffer layer 102 is provided on the (11-20)-face of the sapphire substrate 601. The undoped GaN buffer layer 102 has a thickness of 300 .ANG.. An n-type GaN contact layer 103 is provided on the undoped GaN buffer layer 102. The n-type GaN contact layer 103 is doped with Si. The n-type GaN contact layer 103 has a thickness of 3 .mu.m. An n-type In.sub.0.05 Ga.sub.0.95 N layer 804 is provided on the n-type GaN contact layer 103. The n-type In.sub.0.05 Ga.sub.0.95 N layer 804 is doped with Si. The n-type In.sub.0.05 Ga.sub.0.95 N layer 804 has a thickness of 0.1 .mu.m. An n-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 905 is provided on the n-type In.sub.0.05 Ga.sub.0.95 N layer 804. The n-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 905 is doped with Si. The n-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 905 has a thickness of 0.5 .mu.m. An n-type GaN optical guide layer 106 is provided on the n-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 905. The n-type GaN optical guide layer 106 is doped with Si. The n-type GaN optical guide layer 106 has a thickness of 0.1 .mu.m. A multiple quantum well active layer 807 is provided on the n-type GaN optical guide layer 106. The multiple quantum well active layer 807 comprises 7 periods of 25 .ANG.-thick undoped In.sub.0.2 Ga.sub.0.8 N quantum well layers and 50 .mu.-thick undoped In.sub.0.05 Ga.sub.0.95 N barrier layers. A p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is provided on the multiple quantum well active layer 807. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is doped with Mg. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 has a thickness of 200 .ANG.. The p-type Al.sub.0.2 Ga.sub.0.8 N layer 108 is capable of suppressing dissociation and evaporation of indium from the indium gallium nitride layers of the multiple quantum well active layer 807. A p-type GaN optical guide layer 109 is provided on the p-type Al.sub.0.2 Ga.sub.0.8 N layer 108. The p-type GaN optical guide layer 109 is doped with Mg. The p-type GaN optical guide layer 109 has a thickness of 0.1 .mu.m. A p-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 910 is provided on the p-type GaN optical guide layer 109. The p-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 910 is doped with Mg. The p-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 910 has a thickness of 0.5 .mu.m. A p-type GaN contact layer 111 is provided on the p-type Al.sub.0.05 Ga.sub.0.95 N cladding layer 910. The p-type GaN contact layer 111 is doped with Mg. The p-type GaN contact layer 111 has a thickness of 0.2 .mu.m. A p-electrode 112 is provided on the p-type GaN contact layer 111. The p-electrode 112 comprises laminations of a nickel layer and a gold layer. An n-electrode 113 is provided on a part of the n-type GaN contact layer 103. The n-electrode 113 comprises a titanium layer and an aluminum layer. The above gallium nitride based semiconductor laser is disclosed in Applied Physics Letters 69, 1996, 1477.
Generally, the gallium nitride based semiconductor laser has a larger gain in transverse electric mode than that in transverse magnetic mode. The emission is made in the transverse electric mode of such an order that the optical confinement coefficient into the gain region becomes maximum.
FIG. 7 is a graph illustrative of optical intensity and refractive index versus a depth from the surface of the semiconductor layer in a gallium nitride based semiconductor laser device of FIG. 4. Individual refractive indexes of every layers in the form of the laser are shown. The optical intensity represents the distribution of light which is calculated in the tenth ordered mode wherein a total value of every optical confinement coefficients into the every quantum well layers becomes maximum. The lowest ordered mode namely the basic mode is zero-ordered mode. The calculation is made assuming that refractive indexes of air and electrodes are 1 and a refractive index of sapphire substrate is 1.79. Since the n-type AlGaN cladding layer 405 is thick, it does not appear that the light is largely distributed into the cladding layer. Notwithstanding, the aluminum index of the cladding layer 405 is relatively high, for example, 0.15, for which reason a probability of generation of crack in the cladding layer is also high. Namely, the crack is likely to be caused in the cladding layer with high aluminum index.
The first, second and third conventional gallium nitride based semiconductor lasers shown in FIGS. 1, 2 and 3 have the same problems as described above. The aluminum index of the cladding layer is high, for example, in the range of not less than 0.12 to not more than 0.15, for which reason a probability of generation of crack in the cladding layer is also high. Namely, the crack is likely to be caused in the cladding layer with high aluminum index.
The first, second and third conventional gallium nitride based semiconductor lasers shown. in FIGS. 1, 2 and 3 also have an InGaN layer which is provided for prevention of crack in the cladding layer. However, the InGaN layer has a large optical absorption loss. This raises a problem with increase in emission threshold current of the laser.
FIG. 8 is a graph illustrative of optical intensity and refractive index versus a depth from the surface of the semiconductor layer in a gallium nitride based semiconductor laser device of FIG. 5. Individual refractive indexes of every layers in the form of the laser are shown. The optical intensity represents the distribution of light which is calculated in the fourth ordered mode wherein a total value of every optical confinement coefficients into the every quantum well layers becomes maximum. The lowest ordered mode namely the basic mode is zero-ordered mode. The calculation is made assuming that refractive indexes of air and electrodes are 1 and a refractive index of sapphire substrate is 1.79. The aluminum index of the cladding layer is relatively low, for example, in the range of not less 0.05 to not more than 0.07, for which reason a probability of generation of crack in the cladding layer is not so high. Namely, the crack is unlikely to be caused in the cladding layer. Since, however, the aluminum index of the cladding layer is relatively low, the optical confinement is insufficient and the light is largely distributed into the n-type gallium nitride contact layer. This presumption of large distribution of light into the contact layer is somewhat different from the fact because the indium gallium nitride layer provided for the prevention of the crack is capable of absorb the light whereby this absorption of light by the indium gallium nitride layer prevents the light from distribution into the contact layer.
However, the absorption of light by the above indium gallium nitride layer causes a large optical absorption loss thereby raising a problem with the increase in emission threshold current of the laser.
Even if, in order to settle this problem, no InGaN layer is provided, then the light is allowed to be largely distributed into the gallium nitride contact layer. This means that the optical confinement coefficient is low and a high emission threshold current is needed. Further, even if the laser beam is condensed by a lens, a spot size of the condensed laser beam is large.
FIG. 9 is a graph illustrative of optical intensity and refractive index versus a depth from the surface of the semiconductor layer in a gallium nitride based semiconductor laser device of FIG. 6. Individual refractive indexes of every layers in the form of the laser are shown. The optical intensity represents the distribution of light which is calculated in the fourth ordered mode wherein a total value of every optical confinement coefficients into the every quantum well layers becomes maximum. The lowest ordered mode namely the basic mode is zero-ordered mode. The calculation is made assuming that refractive indexes of air and electrodes are 1 and a refractive index of sapphire substrate is 1.79. The aluminum index of the cladding layer is relatively low, for example, in the range of not less 0.05 to not more than 0.07, for which reason a probability of generation of crack in the cladding layer is not so high. Namely, the crack is unlikely to be caused in the cladding layer. Since, however, the aluminum index of the cladding layer is relatively low, the optical confinement is insufficient and the light is largely distributed into the n-type gallium nitride contact layer. This presumption of large distribution of light into the contact layer is somewhat different from the fact because the indium gallium nitride layer provided for the prevention of the crack is capable of absorb the light whereby this absorption of light by the indium gallium nitride layer prevents the light from distribution into the contact layer.
However, the absorption of light by the above indium gallium nitride layer causes a large optical absorption loss thereby raising a problem with the increase in emission threshold current of the laser.
Even if, in order to settle this problem, no InGaN layer is provided, then the light is allowed to be largely distributed into the gallium nitride contact layer. This means that the optical confinement coefficient is low and a high emission threshold current is needed. Further, even if the laser beam is condensed by a lens, a spot size of the condensed laser beam is large.
In the above circumstances, it had been required to develop a novel gallium nitride based semiconductor laser free from the above problems.